Semiconductor integrated circuit dies typically are mounted onto printed circuit boards via a plurality of interconnections between solder bumps, which are affixed to the face of the die, and contact pads that are on a surface of the printed circuit board. The contact pads are in electrical communication with one or more circuit lines that are also present on the surface of the board. Typically, the contact pads and circuit lines are integrally formed from a single layer of copper.
To form the interconnections, a solder mask is first applied to the surface of the printed circuit board. The solder mask, which acts as an insulator, is formed from a polymeric material that is able to withstand the elevated temperatures to which it is exposed during subsequent assembly operation. Openings are formed in the solder mask to expose the upper surface of each of the contact pads. Thereafter, the solder bumps are aligned with the openings and brought into contact with the contact pads. The resulting assembly is heated to a temperature sufficient to melt the solder bumps and cause them to reflow over the corresponding contact pads. The presence of the solder mask prevents the molten solder from spreading along the circuit lines. Upon cooling, the solder resolidifies and bonds to the contact pad to complete the interconnection.
Unfortunately, it is very difficult to routinely form good interconnections between all of the solder bumps and contact pads using the above-described method. Poor electrical and mechanical connections often result from misalignment of the solder mask with the contact pads so that the contact pads are not centered in the openings. Poor connections can also result if the solder bumps are not all properly aligned with the openings in the solder mask. Because the solder bumps typically have non-uniform heights and because the solder mask itself sometimes has a non-uniform thickness, poor connections can result even when the contact pads, openings, and solder bumps are all properly aligned.
Electrical connections between printed circuit boards and semiconductor integrated circuit dies have also been achieved using conductive adhesives. Since conductive adhesives are considerably more viscous than molten solder, a solder mask is not needed. However, it is necessary that the printed circuit board be spaced apart from the die for a distance sufficient to prevent shorting between the die and circuitry and to provide stress relief. To achieve such distances conductive bumps must be formed on the surface of the printed circuit board. Unfortunately, poor interconnections can result if the upper surfaces of the bumps do not extend to essentially the same distance above the surface of the printed circuit board. In other words, poor interconnections can result if the upper surfaces of the bumps are not substantially co-planar.
Accordingly, it is desirable to have a new printed circuit board that overcomes these problems. A printed circuit board configured to provide a high density of uniformly good interconnections between the printed circuit board and the semiconductor integrated circuit die is especially desirable.